Wednesday, 13 August 2014

Isotopes add to ‘Anthropocene’ debate... by Jonathan Dean

Are we in a new geological age?Have we really altered the global environment? When did these impacts reach a critical point?

These are just some of the questions that Jonathan Dean, Melanie Leng and Anson Mackay have attempted to answer in a major new piece of research. Here Jonathan tells us more about their research and why you might have been born in a different geological age to your grandparents… Our paper has just been published in The Anthropocene Review, which is available for free here. It stemmed from the increasing debate in the geological and wider scientific communities regarding whether a new geological age called the Anthropocene should be defined. At the moment, the Anthropocene is an informal term that denotes the impact humans have had on the Earth. It is argued that, since humans are the dominant force of global environmental change, it is no longer appropriate for us still to be in the same geological age as when humans were living in caves and not flying round the world emitting vast amounts of CO2. A working group of the International Commission on Stratigraphy is set to present its preliminary findings in 2016 on whether a change should be made to geological time. The major sticking point is likely to be where to set the beginning of the Anthropocene, with some people arguing for thousands of years ago when humans started chopping down forests and farming, to the last few centuries with the Industrial Revolution, to sometime in the future when greenhouse gas emissions are predicted to lead to large temperature increases. As isotope geochemists, we decided to review how isotope records can help with this debate. Some details if you are still with me! Isotopes are different types of an element: they have the same number of protons but a different number of neutrons. The ratio of one isotope of an element to another will change in response to human impacts on the environment, so isotope ratios can be used to establish how humans have altered the global environment and when changes began to be momentous. We can use carbon isotopes (carbon-13 and carbon-12) to investigate human impact on the composition of the atmosphere. Fossil fuels (coal, oil and gas) are the remains of organisms that lived millions of years ago, and because organisms preferentially use carbon-12 rather than carbon-13 when they grow, when we burn fossil fuels this releases large amounts of carbon-12 into the atmosphere. The increase in carbon-12 in the atmosphere over the last few centuries as we have burnt large amounts of fossil fuels can be reconstructed by analysing gas bubbles that are locked away in ice sheets in Greenland and Antarctica. They show there was a trend to increasing carbon-12 (indicating increased CO2 emissions due to humans burning fossil fuels) since the Industrial Revolution, but that there was a big acceleration in this trend after the Second World War when economic growth took off.

Another example is that of lead isotopes. These can be used as a fingerprint to trace the sources of lead pollution, since human mining and smelting release lead with different isotope ratios to natural processes. This has allowed researchers not only to show that lead pollution found in ice from Greenland dating to 2,000 years ago was due to human, not natural processes, but that it was lead released by mining and smelting of Spanish lead by the Romans!

Overall, in the paper we demonstrate that isotopes show substantial human impacts on the environment. While we showed that different isotopes show different impacts at different times in the past, there is a clear acceleration in the input of CO2 into the atmosphere (recorded by carbon isotopes) around 1950. Other indicators, such as deforestation and species extinction rates, also suggest this was a time when human impacts on the environment increased substantially. If I were a betting man I’d probably go for the start of the Anthropocene being fixed around this time, which would mean that many of us were born in a different geological age to our grandparents!